Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder engine can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
In the deactivated mode, there are fewer cylinders operating. As a result, there is less drive torque available to drive the vehicle driveline and accessories (e.g., alternator, coolant pump, A/C compressor). Engine efficiency is increased as a result of decreased fuel consumption (i.e., no fuel supplied to the deactivated cylinders) and decreased engine pumping. Because the deactivated cylinders do not take in and compress fresh intake air, pumping losses are reduced.
Deactivated mode operation may be restricted to avoid the noise, vibration, and harshness (NVH) resulting when the engine speed matches the torque converter turbine speed. As a result, overall engine efficiency is decreased to avoid NVH. The engine speed is generally more likely to match the torque converter turbine speed at lower engine speeds than at higher engine speeds. Traditionally, deactivated mode is more restricted at lower engine speeds. Increasing the slip speed of the torque converter clutch to avoid NVH may result in high torque converter clutch temperatures and damage to the friction surfaces of the torque converter clutch.